1. Field of the Invention
This invention relates to a method for producing filament yarns of high strength, high modulus carbon fibers. More particularly, it relates to a melt-spinning process of filament yarns of high strength, high modulus carbon fibers in which a heated inert gas or a non-oxidative gas is fed steadily on the surfaces of spinning nozzles to stabilize and ease the melt-spinning.
2. Description of the Prior Art
As the result of recent rapid growth of industries for manufacturing aircrafts, motor vehicles and other transport, a demand for materials prepared by a combinaton of special materials as a material necessary to these industries capable of exhibiting remarkable characteristics because of the superiority of some of their physical properties is ever increasing. Particularly, the demand for the advent of inexpensive materials provided with high strength and high modulus together with lightness of weight is great. However, since the material which satisfies the above-mentioned demand cannot be supplied in a stabilized manner according to the present status of art, research works relative to composite articles (reinforced resins) which meet the above-mentioned requirement are prevailing.
As one of the most promising material to be used as reinforced resin, there can be mentioned high strength high modulus carbon fibers. These materials have appeared from about the time when the rapid growth of the above-mentioned industry just started. When the carbon fibers are combined with a resin, it is possible to produce reinforced resins capable of exhibiting characteristic feature unparalleled in the past. To be regretful enough, however, in spite of the high strength and high modulus of the carbon fibers for the above-mentioned reinforced resins capable of exhibiting extremely notable characteristic feature, the application fields of these fibers have not expanded. The cause of this fact, as explained later, lies in the higher production cost.
It is well known that the material for high strength, high modulus carbon fibers which are commercially available are mostly polyacrylonitrile fibers produced by a special production process and a special spinning process but these acrylonitrile fibers are not only expensive as a precursor of carbon fibers but also the production yield thereof from the precursor is as low as less than 45%. These facts complicate the treatment steps and enlarge production facilities for producing superior carbon fibers, resulting in the increasing production cost of the ultimate products of carbon fibers. The production cost of high strength, high modulus carbon fibers of the ultimate product is further increased by the treatment cost, etc. of hydrocyanic acid by-produced at the time of carbonization treatment.
As for one method for producing high strength, high modulus carbon fibers at a low cost, there are descriptions in the official gazettes of Japanese patent publications No. 1810 (1979) issued to Union Carbide Corporation, and it is a well known fact that mesophase-containing pitches are extremely superior raw material as raw materials for filament yarns of high strength, high modulus carbon fibers. For pitches as raw materials of high strength, high modulus carbon fibers, the content of mesophase and the physical properties of mesophase itself naturally give large influence upon the physical properties of carbon fibers. The higher the mesophase content and the better the quality of mesophase, the greater the improvement of the physical properties of carbon fibers. Namely, for producing high strength, high modulus filament yarns of carbon fibers having superior quality, the purity of mesophase and uniformity of its quality are indispensable condition. However, as a matter of fact, it is difficult at the present status of art to improve the purity and quality of mesophase necessary for producing superior filament yarns of high strength, high modulus carbon fibers to satisfactory extent.
The inventor of the present invention has found that extremely superior filament yarns of high strength, high modulus carbon fibers can be produced by subjecting petroleum-origin or tar-origin pitch which is a residual carbonaceous material produced as a by-product of catalytic cracking (FCC) of vacuum gas oil, to heat treatment at a heating temperature of 360.degree. C. to 420.degree. C. while blowing therein a non-oxidative gas such as hydrocarbons of small carbon numbers to produce a mesophase-containing pitch, subjecting the resultant pitch to heat aging at a heat treatment condition entirely different from that at the time of mesophase preparation, separating mesophase (easily confirmable by using a polarization microscope) by utilizing the difference of physical properties (e.g. specific gravity or viscosity), subjecting the resultant mesophase, as a raw material, to melt spinning, followed by thermosetting and carbonization. However, even the above-mentioned mesophase, which is separated by utilizing only the difference of physical properties (e.g. viscosity or specific gravity) is not accurately uniform in qualities (e.g. molecular weight, chemical structure and other physical properties) depending upon the difference of crude oil as a raw material of petroleum-origin pitch or kind of coal as a raw material of tar-origin pitch. There are Q.I. mesophase (insoluble in quinoline at 80.degree. C.) and Q.S. (measured by extraction with quinoline in a soxhlet apparatus at 80.degree. C.) in mesophase. Even in case of melt-spinning of mesophase, middle or lower molecular weight compounds in mesophase adjere to spinning nozzles, turn by heat to thermoset material due to contact with air, and gaseous components liberated from the spinning nozzles and raw filaments stain the spinning nozzles. When the contamination is built up, spinning of raw filaments becomes unstable, causing frequent breakage of filaments which gives extremely harmful effect on qualities and production.
It is an object of the present invention to provide a method for producing filaments yarns of high strength, high modulus carbon fibers in which the above-mentioned drawbacks of the conventional art have been overcome according to the method of the present invention.